The present invention relates to a planar member for a shadow mask of a cathode-ray tube and a method for manufacturing a shadow mask using the planar member.
As illustrated in FIG. 7, a color cathode-ray tube 1 with a flat face includes a glass panel 2 having a substantially flat face screen 3. Panel pins 5 are attached to the inner surface of the glass panel 2, for supporting a metal frame member 7 for a shadow mask 9. Plate springs 8 attached to the outer surface of the frame member engages the panel pins 5, so that the shadow mask 9 stretched on the frame member 7 is fixed at a predetermined position in the glass panel 2. Electron beams 10 emitted by an electron gun (not shown in the Figure) pass apertures of the shadow mask 9 and hit corresponding spots of a fluorescent screen 4 formed on the inner face of the glass panel 2.
Some electron beams do not pass the apertures but hit the shadow mask 9. Consequently, the shadow mask is heated and may suffer a thermal expansion. If the shadow mask is deformed by the thermal expansion, color purity may be deteriorated when the electron beam mishits a proper spot of the fluorescent screen after passing through the aperture at a deformed portion of the shadow mask. In order to suppress the deformation of the shadow mask due to the thermal expansion, the shadow mask 9 is usually given a certain tension by welding while being stretched on the frame member 7.
In the prior art, a planar member 21 shown in FIG. 8 was used for stretching and welding the shadow mask 9 on the frame member 7. This planar member 21 includes a beam-selecting area 22 with a lot of small slots or apertures for passing an electron beam, which are disposed at a predetermined pitch and pattern, and an area 23 without apertures surrounding the beam-selecting area 22. The beam-selecting area 22 has a substantially rectangular shape to be a shadow mask.
The beam-selecting area 22 is positioned on the frame member 7, and a tension is applied to the beam-selecting area 22 by pulling the four sides of the planar member 21, i.e., the area without apertures. Then the planar member is welded to the frame member 7. The area without apertures 23 is cut off along the outer rim of the frame member 7. Thus, a shadow mask assembly, i.e., a color selecting electrode is manufactured.
As mentioned above, the beam-selecting area has a lot of apertures and the surrounding area 23 has no aperture. Therefore, there is a big difference in tensile strength between the beam-selecting area 22 and non-aperture area 23. Consequently, when the four sides of the planar member 21 are pulled outward by a predetermined force, the extension amount is different between the beam-selecting area 22 and the non-aperture area 23, resulting in a wrinkle being generated at corners (24 in FIG. 8) of the beam-selecting area. The wrinkle in the beam-selecting area 22, which is to be a shadow mask, may cause a deterioration of color purity since correspondence of the beam passing aperture and the spot on the fluorescent screen may shift at the wrinkled portion.
Another problem of the prior art is that the boundary portion of the beam-selecting area 22 and the non-aperture area 23 has a tendency to break when the four sides of the planar member 21 are pulled to the outside. As a result, it is difficult to raise a yield rate in the manufacturing process.